I can never just stop at naked, I make them do circus tricks and all sorts of crazy stuff. Essentially making me their audience, it works well for the nerves too. Not so much for the presentation, it's pretty distracting.

I usually picture them in white and red dotted underwear, but that soesn't help much. Except i'm not nervous anymore but i get giggly-fits. xDBut once we had a presentation about various subjects on medicine, poisons and drugs and the group before me had baldrian. They also gave some around on some sugar. When it was my turn (i had Mushroom poisons) the Teacher told me to take another one!! My presentation went well, but i felt really drugged and told them so. They just shook it of, that it was fine. (Was my best presentation ever )